Robert -
Please refer to the below. It is assumed you meant to reference CSA/UL60950 for power cross requirements, rather than IEC 60950. It is also assumed you meant Figure 6C, rather than Figure 5C. The below is somewhat stream of consciousness; please forgive the format of the response. > From: Robert Johnson > Sent: Wednesday, November 26, 2003 7:27 AM > > When using coaxial cable for TNV-1 purposes, I > have trouble figuring out how to apply clause 6. > A typical example is “fiber to the curb” where > the final service drop from the pole to the subscriber > is made with small coax such as T3 lines. I'll assume T3 is a purely figurative example. The bulk of fiber-to-the-curb I've seen (it has been a little while) were intended to provide POTS, Ethernet, CATV type (movies-on-demand), broadband and similar services, mostly to small businesses and residential subscribers. T1, T3, etc., might be offered now, but weren't among the services contemplated when Mr. Peabody took his boy Sherman through the Wayback Machine. > The cable shield is grounded at the equipment > 6.1 can be passed by having the serviceman ground the > equipment (6.1.2.2). Per US NEC, Article 800.33, the cable shield must also be grounded, as close as practicable to the point of entrance to a building. See also CEC, Rule 60-306. > 6.2 can be passed by grounding both TNV and SELV. And the grounding meets the requirements of §2.6. > 6.3 is passed since no power is applied to the > TNV circuit. Yes. > 6.4 is a problem however since primary and secondary > protectors are not used. Do you intend to mean primary protection in the equipment or at the building entrance? It is not very common, but sometimes so, that primary protection is incorporated into equipment. The assumption of the standard is that primary protection is present, based on Code requirements (NEC 800.12 and CEC Rule 60-200). Secondary protectors are not required by the Codes or the standard. The requirements for secondary protectors are related to the 100 A²·s current limiting and 1.3 A overcurrent protection requirements in the left hand column of Figure 6C; not as a "requirement," but "considered to meet" the requirement. > The central path of figure 5C can be used for > compliance since the coax is greater than 26 AWG, > the circuit is grounded, and an enclosure and > spacings are provided. Here is where divergence from 100 A²·s current limiting occurs, moving to the middle path of Figure 6C... > However, this does not solve the problem of a power cross > where up to 25 amps (-or more; I believe this figure came > from the twp lines) may be carried into the building to > the equipment on the coax, overheating the telecom cable. Here is where divergence from the middle path occurs. One must assume the product does not provide the 120V isolation anticipated by Note 4. Either §6.1.2 and Figure 6A testing, or a simple EST demonstrates compliance and the middle path still applies... §6.1.2 assumes overvoltage protection exists in the equipment, but if it does not, an EST meets the middle path requirement. You say spacings are met, so this would be easy... If you have to go through Test 5, you will need some form of overcurrent protection, all of the circuit elements must be robust or there is some inherent circuit impedance at 60 Hz to limit current flow (the latter is unlikely for high speed digital circuits). In this case, §NAC.2.3 doesn't require use of a wiring simulator, but if the circuit doesn't contain overcurrent protection, the product may not comply. > Have I overlooked some requirement or does the standard > avoid addressing this problem? Is there a suggested > solution: The only one I can think of is adding a ground > at the service entrance and fusing the shield, but this > is not a current practice. > > Bob Johnson > ITE Safety Fusing the shield doesn't make sense. Fusing the center conductor does. Regards, Peter L. Tarver, PE [email protected] This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ieee-pses.org/ To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: [email protected] Dave Heald: [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
